Disclosing the formation of ring-banded spherulites for semicrystalline polymers through the double-layer film method

The formation of ring-banded spherulites in double-layer semicrystalline polymer films has seldom been a concern. In this study, the nucleation and growth of ring-banded spherulites in poly(ethylene oxide)/poly(L-lactide) (PEO/PLA) double-layer films during isothermal crystallization at various temperatures above the melting point of the PEO layer have been investigated by using polarized optical microscopy (POM). The sequential crystallization of PLA and PEO components in the double-layer films was confirmed by a double quench crystallization procedure. The surface morphologies of ring-banded spherulites were examined by using scanning electron microscopy. The experimental results are compared with those for PLA/PEO blend films with 25 wt% PEO composition. It is interesting to find that the changing trend of band space in ring-banded spherulites at different isothermal crystallization temperatures in PEO/PLA double-layer films is more complex than that in PLA/PEO blend films. For the former ones, the ring-banded spherulites form at the lowest temperature approaching 65 degrees C and the band space slightly increases from 65 to 90 degrees C, is absent from 90 to 105 degrees C, obviously decreases from 105 to 115 degrees C, and significantly increases from 115 to 135 degrees C. For the latter ones, the band space only shows a monotonic increase with increasing temperature at above 105 degrees C. The mechanism for the formation of ring-banded spherulites in the PEO/PLA double-layer film system is proposed. The experimental results may shed some light on explaining the formation mechanism of ring-banded spherulites in semicrystalline polymers.